supplement necessary instructions to run example-wasi (#4633)

examples/wasi/main.c

@@ -3,6 +3,7 @@ Example of instantiating a WebAssembly which uses WASI imports.
 
 You can compile and run this example on Linux with:
 
+   cmake example/
    cargo build --release -p wasmtime-c-api
    cc examples/wasi/main.c \
        -I crates/c-api/include \

examples/wasi/main.rs

@@ -1,7 +1,11 @@
 //! Example of instantiating of instantiating a wasm module which uses WASI
 //! imports.
 
-// You can execute this example with `cargo run --example wasi`
+/*
+You can execute this example with:
+    cmake example/
+    cargo run --example wasi
+*/
 
 use anyhow::Result;
 use wasmtime::*;